The present invention relates to a shift arrangement for motor vehicle power transfer devices and, more particularly, to an improved sector plate incorporated into a transfer case shift mechanism.
In general, power transfer mechanisms, such as transfer cases, are operatively associated with both manual and automatic transmissions for selectively directing power to the non-driven wheels of a motor vehicle upon shifting from a Two-Wheel drive mode to a Four-Wheel drive mode. Modernly, conventional transfer cases use a synchronizer clutch arrangement in association with a first shift fork mechanism to provide "shift-on-the-fly" Two-Wheel drive to Four-Wheel drive mode shifting. When shifting the transfer case between the Two-Wheel drive and Four-Wheel drive operating modes, selective actuation of a shift lever by the vehicle operator causes corresponding rotation of a sector plate. In particular, a first cam follower or "mode" pin is secured to the first shift fork mechanism for causing axial movement thereof. Typically, the mode pin is biased into engagement with a contoured outer edge surface of the sector plate. As such, the profile of the outer contoured edge causes the mode pin and, in turn, the first shift fork mechanism to be axially moveable between positions defining the Two-Wheel drive mode and the Four-Wheel drive mode in response to rotation of the sector plate.
Most automotive transfer cases are typically equipped with a speed reduction gear assembly or the like for providing "High" and "Low" speed ranges in conjunction with the Two-Wheel drive and Four-Wheel drive operating modes. As such, a second shift fork mechanism is provided for operably shifting components of the speed reduction gear assembly between a High-Range position providing direct drive and a Low-Range position providing reduction ratio drive. Conventionally, a second cam follower or "range" pin is secured to the PG,3 second shift fork mechanism and extends into a contoured range slot formed in the sector plate. The range slot defines a continuous cam track within which the range pin travels along a common bi-directional pathway. Engagement between the range pin and the continuous cam track causes axial movement of the range pin and, in turn, the second shift fork between the High-Range and Low-Range positions in response to rotation of the sector plate.
One example of a transfer case incorporating a conventional range pin and slot arrangement on a sector plate is shown in the U.S. Pat. No. 4,770,280 issued to Frost on Sep. 13, 1988 and entitled "Snap-Action Arrangement For Transfer Case Synchronizer". The cam track formed by the range slot includes an upper "mode" section and a lower "range" section. Each of the "mode" and "range" sections of the range slot are defined by substantially parallel inner and outer arcuate edges having a common radius that are laterally offset to provide only minimal clearances with the range pin.
In operation, the "mode" section of the range slot is configured to substantially maintain the axial position of the range pin (and the second shift fork mechanism) while permitting rotation of the sector plate between the Two-Wheel drive mode and Four-Wheel drive mode positions. However, the "range" section of the range slot is configured such that engagement between the range pin and one of the inner and outer arcuate edges causes a predetermined amount of axial movement of the range pin and the second shift fork in a first direction upon rotation of the sector plate from the Four-Wheel High-Range position to the Four-Wheel Low-Range operative positions. Likewise, rotation of the sector plate from the Four-Wheel Low-Range position to the Four-Wheel High-Range position causes the engagement between the range pin and the other of the inner and outer arcuate edges which, in turn, generates a predetermined amount of axial movement of the range pin and second shift fork in a second direction. Therefore, the contour of the laterally offset inner and outer arcuate edges associated with the "range" section determines the magnitude of the axially directed forces exerted on the range pin for a given input torque applied to the shift lever. Accordingly, such conventional range shift arrangements having common radius edge profiles define a common bi-directional pathway along which the range pin travels within the "range" section of the range slot during rotation of the sector plate in both directions. As a result, the axial force versus pin displacement characteristics generated in the "range" section are likewise bi-directional for shifting into and out of either range position. Accordingly, while conventional range shift arrangements generally perform satisfactorily, a compromise typically exists between maximizing shift forces and optimizing shift feel.